Have you checked the nch model? As I said before there might be a statement in the model file that shows which section is referring to what kind of device. It'd probably in the first actual line in the model declaration and it'd probably be limiting width or length of the devices for each section in the model. When a parameter is passed to it, the model file will use parameters from the respective section.
In the netlist you should see exactly which model file is included but not the logical decisions made inside the model file, and in the CDF you can see how the model file is associated with the symbol you're using. I assumed you were asking more than one thing.
Edit: Ok, I shook off my laziness and checked out the TSMC018 model I had access to. You should look for LMIN, LMAX, WMIN, WMAX statements. They are the parameters that define which one is being used. And for a little bit more information check out **broken link removed**